1. Field of the Invention
This invention relates to actuators and, more particularly, to actuators powered by shape memory alloy (SMA) wire.
2. Description of Related Art
U.S. Pat. No. 6,326,707 discloses linear actuators that are driven by shape memory alloy (SMA) materials, and feature stroke amplification by multiple bars or rods (sub-modules) linked together by SMA wires. In these and other SMA mechanisms, it has been understood that a restoring force is necessary to return an SMA wire from its heated (contracted) state to its cooled (extended) state. Many prior art SMA actuator designs have made use of common spring assemblies, such as helical or leaf springs, to exert the required restoring force. These spring assemblies typically deliver a spring force that varies linearly with displacement, (F=kx), and the restoring force in most cases is a maximum at maximum stroke. It has been found that the SMA component responds poorly to this force/displacement characteristic, and the useful life of the SMA actuator is severely limited by such a restoring force. The patent referenced above describes several spring arrangements that deliver variable restoring force (variable, or inverse linear, or the like) to optimize the performance of the SMA components.
It is apparently not widely known that some commercially available SMA wires, due to well-understood material processing steps, have the ability to return completely to their original shape without application of an external restoring force. This behavior is termed the reversible shape memory effect. The force produced as the wire cools and returns to its quiescent length is very small; that is, a small fraction of the useful force produced when it contracts upon heating. It is not practical to make a device that produces usable force on the return stroke as well as the forward stroke. One reversible shape memory device in the prior art is a helical spring that expands lengthwise upon heating, and contracts fully to its quiescent length upon cooling. There appears to be no other devices in the prior art that exploit the reversible shape memory effect to useful effect.